The present invention relates to secondary galvanic cells with alkali anode and non-aqueous electrolyte solution in general and, more particularly, to those employing a cathode material which allows for the repeated recharging of the cells.
A considerable number of secondary cells with an alkali anode and a non-aqueous electrolyte solution have been described. These employ as cathode materials the dichalcogenides, i.e. the dioxides, disulfides, diselenides and ditellurides, of the transition metals of the IV B, V B, and VI B groups of the Periodic Table of the Elements. All of these compounds have a layered crystal structure and permit repeated reversible intercalations of alkali metal ions, whereby their lattice parameters change only to an insignificant extent. The best electrochemical performance thus far reported in the scientific literature is achieved with Li cells using stoichiometric TiS.sub.2 as a cathode material. However, the synthesis of this material is time consuming and difficult. Moreover, the pure titanium sponge employed as a starting material is rather expensive.
Chromium is a more easily available transient metal. However, all attempts to synthesize CrS.sub.2 directly have hitherto failed. A more successful approach to the obtention of a stable, layered compound based upon Cr, which can reversibly intercalate alkali metal ions and, more particularly, those of lithium, is the use of sodium thiochromite, i.e. NaCrS.sub.2, from which the non-stoichiometric compound, i.e. Na.sub.y CrS.sub.2 (0.1&lt;y&lt;0.4) is obtained by means of electrochemical or chemical oxidation, thereby providing sites for the reversible intercalation of Li. The non-stoichiometric sodium thiochromite, Na.sub.0.2 CrS.sub.2, is likewise a layered compound, possessing the same stable crystalline structure as the stoichiometric species, thereby permitting repeated electrochemical recharging in lithium cells with non-aqueous electrolyte solutions. However, this compound is characterized by a very low electronic conductivity, which strongly hinders the diffusion of the alkali ions in the crystal lattice of the host. The electron transfer from the cathode material to the current collector is also retarded. This imposes the use of a larger amount of electroconductive additives, such as e.g. graphite or carbon black. The result is that the permitted current drains are limited to approximately 1 mA/cm.sup.2.